For example, the pigmentation, the freckle, the melasma, and the senile lentigo, which are caused on the skin after the suntan, reside in such a state that the melanin production is extremely facilitated on account of the activation of the pigment cell (melanocyte) in the skin. The component, which is well-known to have the function for preventing or ameliorating the onset and worsening of the skin pigment trouble as described above, is the compound (skin whitening agent) having the skin whitening function including, for example, ascorbic acid derivatives, hydrogen peroxide, colloidal sulfur, glutathione, hydroquinone, and catechol (see, for example, Non-Patent Document 1 and Non-Patent Document 2). External preparations for skin, which are blended with the components as described above as the active ingredients, are widely used. At present, a variety of mechanisms of action have been reported as the mechanism of action possessed by the compound known as the skin whitening agent, including, for example, the tyrosinase enzyme inhibiting action, the tyrosinase-related protein degradation, and the melanin transfer inhibition caused by the suppression of dendrite elongation in melanocyte. Target molecules are present with respect to the respective mechanisms of action. In order to express the high skin whitening effect, an organic low molecular weight compound, which appropriately interacts with the target molecule, is useful. Further, the organic low molecular weight compound, which appropriately interacts with each of the target molecules, has the structural characteristic which differs depending on each of the target molecules. Therefore, studies are vigorously performed as well in relation to the optimization of the chemical structure in order to maximally make the use of the pharmacological action exhibited by the organic low molecular weight compound. At present, studies on the skin whitening agent are not limited to the compound which has the high efficacy and the high selectivity with respect to the existing target molecule, and studies are widened, for example, to the compound which simultaneously acts on a plurality of skin whitening target molecules and the compound which has a novel mechanism of action. The high skin whitening action is expected for the skin whitening agent as described above. Actually, the screening has been carried out in relation to compounds having excellent skin whitening functions while seeking for useful compounds which have various chemical structures or pharmacological characteristics. Any skin whitening agent, which has a novel scaffold, is still demanded even now.
Proteins, which constitute the living body, are composed of 20 types of α-amino acids having different side chains which are called “essential amino acids” in almost all cases. Various biological activities have been reported for α-amino acids described above in addition to the function to constitute the biological components. Further, methionine and cysteine, which are included in α-amino acids described above and each of which has the sulfur atom in the chemical structure, are expected to have any biological activity resulting from the characteristic of the sulfur atom differently from the other α-amino acids, and they are applied to various fields including, for example, pharmaceuticals, cosmetics, and foods. In particular, in the field of the cosmetics, the use has been reported, for example, as a reducing component for hair (see, for example, Patent Document 1) and a moistening component (see, for example, Patent Document 2) in relation to external preparations for skin blended with methionine and cysteine. N-Acetyl-L-cysteine, which is a cysteine derivative, is metabolized into glutathione which is an antioxidant. Therefore, N-acetyl-L-cysteine is used as a supplement. However, when such an α-amino acid derivative, which includes the sulfur atom in the molecular structure, is applied, for example, to a cosmetic preparation, a problem arises, for example, such that the amino acid itself or a composition such as an external preparation for skin are unstable, and an unpleasant smell is caused by decomposition products thereof. On the other hand, it is known that cysteic acid and any derivative having an aliphatic acyl group on its nitrogen atom involve, for example, the oil-soluble base material (see, for example, Patent Document 3), the surfactant (see, for example, Patent Document 4), the mucus dissolving activity, and the antioxidizing function (see, for example, Patent Document 5). Further, it is known that N-benzyl derivative of cysteic acid involves the antioxidizing function (see, for example, Patent Document 6). However, as far as the present inventors know, it is not known that the compound represented by the general formula (1) described above, the isomer thereof, and/or the pharmacologically acceptable salt thereof have/has the prophylactic or ameliorating action for pigmentation. Further, as far as the present inventors know, it has not been known that the concerning compound has the good solubility in the hydrophilic or lipophilic medium, the compound is extremely stable in the forms of the compound and the pharmaceutical preparation, and the compound hardly causes any unpleasant smell when the compound is used into a cosmetic preparation such as an external preparation for skin or the like.